In recent years, the use of SMs have become increasingly commonplace. For example, SMs are routinely utilized by magnet resonance imaging ("MRI") systems for imaging structures internal to the human body. Typically, an SM includes a magnet coil composed of superconducting wire that is immersed in a cryogenic fluid which maintains the coil at its superconducting temperature. The widespread use of SMCs in MRI systems is due to the ability to offer a combination of high field strength, low power consumption and relatively low mass.
Outside the field of medical MRI systems, the use of SMCs can be more problematic. For example, for some applications, the size and weight of typical SMC and the use of cryogenic fluids may be unacceptable. Applications of this type include mobile applications, such as shipboard and airborne applications.
A number of designs have attempted to overcome the difficulty associated with the use of SMCs in demanding environments. For example, refrigeration devices such as cryocoolers have been used to conduction cool SMCs to the required operating temperature, i.e., typically less than 10 degrees Kelvin.
However, conduction cooled SMCs have proven to be problematic. In order to operate satisfactorily and achieve the required low operating temperatures, the cooling requirements of the cryocoolers must be minimized. Existing designs have not been able to minimize all sources of heat in and around the SMC and thermally isolate the SMC to minimize the cooling requirements of the cryocooler. Thus, the cryocooler is often unable to cool or maintain the SMC at the required operational temperature and the SMC ceases to superconduct and becomes electronically resistive.
Sources of heat in existing designs include: (i) heat caused by eddy currents in the superconducting wire; (ii) heat caused by eddy currents in the support structure(s) which support the superconducting wire and suspend the SMC from a vacuum vessel and from eddy currents generated in thermal buswork; (iii) heat transferred from the vacuum vessel which is at ambient temperature through the cold-to-warm support structure; and (iv) heat created around the superconducting wire and the support structure(s) caused by stress.
It is well known, that by reducing the mass of the SMC, the mass of the support structure(s) can also be reduced and the heat generated by eddy currents and heat transferred through the support structure(s) are also reduced. However, the design of a lightweight SMC has proven to be problematic. Since the mass of the SMC is reduced, the current density of the SMC must increase. Therefore, the internal stresses generated by the magnet field will also increase. This creates a structural integrity problem for the SMC and the associated support structure(s) which must resist the magnetic forces generated by the SMC. It has proven difficult to design a lightweight SMC and associated support structure(s) with adequate strength to withstand these increased magnetic forces and the shock and vibration caused by motion in the case of a mobile system.
Further, the change in temperature causes dimensional changes in the SMC and the support structure(s) which, in turn, may increase internal stress or result in physical repositioning of the SMC itself relative to the surrounding structures. These stresses caused by temperature changes can physically damage the structure of the SMC and the support structure(s) and de-stablize the SMC.
In light of the above, it is an object of the present invention to provide a SMC and support structure(s) which are lightweight, while retaining sufficient strength in the presence of internally produced high magnetic fields and externally induced forces from shock and vibration. It is an also object of the present invention to provide an SMC which is lightweight while retaining sufficient strength for deployment in a variety of environments. It is another object of the present invention to provide a SMC and support structure which reduces internal stress within the SMC and support structure during cooling of the SMC. Yet another object of the present invention is to provide an SMC and cold-to-warm support structure which provides a high degree of thermal isolation for the SMC and enhanced stability. Another object of the present invention is to provide structural and thermal interfaces to the SMC that do not conduct eddy currents and do not result in undesirable heating.